Frequency shift keying (FSK) is a digital modulation technique that encodes digital information by shifting the frequency of a carrier wave. There are different types of FSK including binary FSK, which uses two discrete frequencies to represent binary 1 and 0, and double frequency shift keying (DFSK), which uses four frequencies to transmit two independent data streams simultaneously. FSK modulation can be demodulated using either FM detector demodulators, which treat the FSK signal as an FM signal, or filter-type demodulators, which use optimal filters matched to the FSK signal parameters. The filters are used to detect the mark and space frequencies, and a decision circuit then determines which was transmitted.

1. Frequency Shift Keying

2. Frequency Shift Keying
 Frequency Shift Keying (FSK) is a
frequency modulation scheme (conveys
information over a carrier wave b
varying its instantaneous frequency) in
which digital information is transmitted
through discrete frequency changes of a
carrier wave.
 Frequency shift keying (FSK) is the most
common form of digital modulation in the
high-frequency radio spectrum, and has
important applications in telephone
circuits.

3. Frequency Shift Keying

4. Binary FSK
 Binary FSK (usually referred to simply as FSK) is
a modulation scheme typically used to send
digital information between digital equipment
such as teleprinters and computers. The data are
transmitted by shifting the frequency of a
continuous carrier in a binary manner to one or
the other of two discrete frequencies. One
frequency is designated as the “mark” frequency
and the other as the “space” frequency. The mark
and space correspond to binary one and zero,
respectively. By convention, mark corresponds to
the higher radio frequency. Figure 1 shows the
relationship between the data and the transmitted

5. Binary FSK

6. DOUBLE FREQUENCY SHIFT
KEYING (DFSK)
 DFSK, sometimes called DFS or twinplex, is a
scheme to transmit two independent binary data
streams by shifting the frequency of a single carrier
among four discrete frequencies. Figure 7 shows a
code table to convert two binary bits to one of four
output states. For example, if bit X and bit Y are 1 and
0, respectively, the output state is C. If each of the
four output states (A through D) is assigned to a
corresponding FSK frequency, then it is possible to
transmit any two bits as a single element. Figure 8
illustrates a typical DFSK signal. Because the two
binary channels are independent, they may contain
any combination of synchronous and asynchronous
signals. It is not uncommon to have a teleprinter in
one channel and a Morse code signal in the other. Bit
synchronization between channels is not required.

7. FSK DEMODULATION
 The demodulation methods for FSK can be
divided into two major categories: FM detector
demodulators and filter-type demodulators.

8. FM Detector Demodulators
 The FM detector demodulator treats the FSK
signal as a simple FM signal with binary
modulation. Figure 9 shows a functional block
diagram for an FM detector-type remove out-of-
band interference and then limited to remove AM
interference. The limited signal is FM-detected to
produce a positive output for a mark condition
and a negative output for a space condition. The
raw, detected signal is lowpass-filtered to remove
noise components at frequencies above the baud
rate, and, finally, the decision circuit makes all
positive voltages into binary 1’s and all negative
voltages into binary 0’s.

9. FM Detector Demodulators

10. FM Detector Demodulators
 This type of demodulator was very popular due to
its relative simplicity and its noncritical tuning.
Phase-locked-loop (PLL) demodulators are a
more recent technique, but they have very similar
performances to that of FM detector
demodulators. However, for a small class of
signals specifically designed for PLL
demodulation, PLL demodulators may perform
better than FM detector demodulators. FM
detector-type demodulators are somewhat
complex for FDM work and cannot generally be
used with interleaved FDM signals. They are also
not commonly used with DFSK signals.

11. Filter-Type FSK Demodulators
 Filter-type demodulators attempt to optimally
match the FSK signal parameters to the
demodulator configuration to optimize
demodulator error performance. A simplified
spectrum for filter-type demodulators is shown in
Figure 13. The proper filter design depends not
only on the signal parameters, but also on the
nature of the signal interference. The classic
“matched” filter demodulator is optimal for
coherent FSK in white gaussian noise
interference. Other filter-type demodulators are
used for noncoherent FSK and/or non-stochastic
noise environments.

12. Filter-Type FSK Demodulators

13. Filter-Type FSK Demodulators
 A block diagram of a simple, matched-filter
demodulator for coherent FSK is shown in Figure 14.
In the demodulator, the output of the matched filters is
compared, and if the output from the mark filter is
larger than that from the space filter, a decision is
made that a mark signal was transmitted. Space
detection is similarly performed. A matched filter
demodulator is optimum because its filters are
matched” to the transmitted signal so that their
response to the desired signal is maximized with
respect to their noise response. For white noise, the
optimum filter has an impulse response equal to the
time-reversed, input signaling element. Because
noncoherent FSK is so much more common than
coherent FSK, it is necessary to have a type of
demodulator that does not depend on phase

14. Decision Threshold Selection
 The simplest decision circuit is a voltage
comparator which decides that a mark signal was
received if the output from the mark detector is
greater than the “noise” output from the space
detector. With equal amplitude transmitted signals
and no selective fading, this is the theoretically
optimum decision circuit. The error performance
of this type of decision for binary noncoherent
FSK in white noise is shown in Figure 17. If the
FSK signal is subject to selective fading (Rayleigh
amplitude statistics), the performance of the
demodulator is severely degraded.

15. Decision Threshold Selection